[isabelle] Requirement 'fontconfig'



Dear users,

I'd like to report a finding that I had to make the hard way today.

I'm currently in the process of migrating the AFP submission service to Ubuntu 18.04 LTS. The package 'fontconfig' is not installed any longer in the server edition. When building Isabelle sessions on such a headless server, this results in the output 'InvocationTargetException' printed on the screen.

The underlying issue is that Isabelle tries to load the available fonts (?) and the JDK cannot do that, because 'fontconfig' is missing. Ultimately, there's a NPE somewhere in the 'sun.awt.*' package.

Luckily, the fix is simple: 'apt install fontconfig'.

Here's the corresponding issue in the AdoptOpenJDK repository: <https://github.com/AdoptOpenJDK/openjdk-build/issues/693>.

Cheers
Lars




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